1. Field of the Invention
This invention relates to a method of producing a leather-like sheet with a delicate appearance and a deluxe hand. More particularly, the invention relates to a method of producing a leather-like sheet with a deluxe hand which comprises preparing and fully curing a leather-like sheet consisting essentially of a porous substrate layer, made up of a fibrous material and/or an elastomer and a nonporous elastomer layer coated onto said porous substrate layer, subjecting said sheet to a flexing or graining or crumpling treatment under no tension in a liquid medium, drying the same sheet at a low temperature and under no tension, with or without a coloration layer as formed on said sheet. The invention is further directed to the leather-like products obtainable by the above method.
2. Description of the Prior Art
A leather-like sheet consisting of a fibrous material and a polymer material based on an elastomeric polymer is known as synthetic leather, simulated leather or artificial leather, and has been widely employed as a substitute for genuine or natural leather in footwear, carrying bags, clothing and interior materials. Heretofore, such a leather-like sheet has been manufactured by fabricating a porous substrate from a fibrous material and polymeric material based on an elastomer, applying a coloration layer comprising a pigment and a polymeric component to the substrate and embossing the assembly. The conventional leather-like sheet obtained in the above manner has serious disadvantages in comparison with genuine leather. Thus, such sheet is not only inferior to genuine leather in the brightness of color and the delicacy of appearance but also lacks the flexibility and solid feeling of genuine leather. Notwithstanding the advantages of such a leather-like sheet, such as greater resistance to water, greater ease with which it may be cleaned when soiled, greater ease with which it may be fabricated into end-use products at higher yields, and lesser fluctuations in supply, the aforesaid disadvantages have put such artificial sheet at a considerable disadvantage, price-wise, as compared to genuine leather. Thus, while genuine leather is almost exclusively employed in expensive, quality products; artificial leather is employed only in medium- and low-priced products, thus creating a general impression that the footwear, bags, interior materials, etc. made of simulated leather sheet are low-quality products. It is an essential prerequisite, therefore, to overcome the above disadvantages in order that leather-like sheets may attain a status equal to that of genuine leather in the market place.
To accomplish bright shades, it has heretofore been proposed to superimpose a polymeric layer with a satisfactory dye receptivity on a porous substrate, in lieu of providing a coloration layer comprising a pigment and a polymer (hereinafter such a product will sometimes be referred to as pigmented product), and then dyeing the same by a jigger or pad dyeing method. However, despite the obvious improvement of the brightness of shades over the more conventional pigmented products, the leather-like sheets obtained by the above procedures still have serious disadvantages. Thus, for example, it is difficult to attain a delicate appearance and the product has a harsh or stiffened feeling.
To attain a delicate appearance, the practice is to emboss the sheet with a heating calender or roller engraved with the grained pattern of genuine leather or to "grain" the sheet under heating on a sanforizing or equivalent machine, which is widely employed for the shrinkproofing of fabrics.
Methods have been adapted to impart "grain" to the sheet by the application of an external physical force at a high temperature near the softening point of the elastomer employed, thus taking advantage of the thermoplasticity of the material; however, these methods give rise to the formation of regular surface patterns in the sheet. These methods are considered disadvantageous in that the products tend to appear "artificially-made" and, because the elastomer layer is subjected to elevated temperature and pressure during the process, the air-permeability, flexibility and feeling of the products are adversely affected.
There also are known procedures for improving the so-called feel or hand of the sheet through a chemical treatment, with a softening agent, or a mechanical flexing or crumpling procedure. However, in the chemical method, certain softening agents enable the obtainment of the desired softness but the same agents result in the rubber-like hand. Moreover, in use of the leather-like sheet, such softening agents migrate to the surface of the product causing discoloration, color fading and other defects in addition to the comparatively higher cost of production. In the case of the latter mechanical method, the hitherto-proposed procedure is such that a water-impregnated leather-like sheet is crumpled in an air-containing rotary drum, but generally it is not only difficult to accomplish an adequate graining or crumpling effect (soft feeling) but the disadvantage is encountered that it produces unnecessary wrinkles characteristic of mechanical crumpling. Furthermore, mechanical crumpling, as such, could result in reduced strength if carried out to an excessive degree.